Elucidating the mechanism of how telomere attrition limits cellular proliferation has important implications for the normal human aging process. Progressive telomere shortening induces replicative senescence, and mice possessing dysfunctional telomeres have reduced life spans. These results strongly implicate a role for telomere dysfunction in both cellular and organismal aging. We have studied the telomere binding protein Potla (protection of telomeres 1a) and found that it is a main determinant of the protective states of telomeres. Pot1 is a single-stranded telomere binding protein that is essential for chromosomal end protection and telomere length homeostasis. Using a Potla conditional knockout mouse generated recently in my laboratory, we show that deletion of Potla induces a DNA damage response that triggers a senescence phenotype indistinguishable from replicative senescence. Loss of Potla also results in extensive chromosomal fusions, suggesting that this protein is required to protect the 3'telomeric overhang from inducing p53-dependent cell cycle arrest. In this proposal, we will use the Potla conditional knockout mouse to test the hypothesis that deletion of Potla results in telomere uncapping and activation of the DNA damage response to initiate cellular senescence and premature aging in vivo. In Aim 1, we will test the hypothesis that loss of Potla leads to telomere deprotection to signal components of the DNA damage pathway to initiate cellular senescence. In Aim 2, we will determine whether conditional deletion of Potla in diverse adult tissues results in the onset of cellular senescence, and whether accumulation of senescent cells result in the onset of aging phenotypes in vivo. We will study several aging phenotypes, including longevity, glucose tolerance, bone density, wound healing, and tumor incidence, over the natural lifespan of these mouse cohorts. In Aim 3, we will determine the level of telomere dysfunction and genomic instability in Potla deficient mice to temporally correlate the onset of premature aging phenotypes with elevated telomere dysfunction and DNA damage response. Our proposal should reveal how telomere uncapping impacts upon both cellular and mammalian aging processes. Given increasing evidence that telomere attrition reduces lifespan in humans, we believe our findings will be relevant to human aging as well.